1. Field of the Invention
The present invention relates to the field of defect detection in disk storage systems, and more particularly, to a method and apparatus for providing the profile of defects on the surface of a magnetic disk.
2. Description of the Related Art
There is a significant quality control problem associated with surface imperfections on magnetic disks. This typically occurs, for example, on nickel-plated aluminum substrates used in the manufacture of thin-film magnetic media, but may be a problem with respect to any area where a smooth surface is desired. Typical surface defects include pits, dirt, dust, oil, stains, fingerprints and the like. Defects on the surface of rigid magnetic media are often a result of an impingement onto the surface or a tearing of material away from the surface.
These types of defects can be very large scratches or gouges on the surface or very small (5 .mu.m and smaller) tears or pricks on the surface. The large surface defects, because of their size and scattering properties, are readily distinguishable through sophisticated data processing performed on light reflected from the surface of the medium under test by an inspection apparatus which includes a light source directed at the disk. Small surface defects (5 .mu.m and smaller) have not been so readily detectable, and even when detected, have been difficult to identify and classify.
For example, the systems described in U.S. Pat. Nos. 4,794,264 and 4,794,265, entitled "Surface Defect Detection And Confirmation System And Method" and "Surface Pit Detection System And Method", respectively issued to Quackenbos, describe systems for detecting pits on a smooth surface by irradiating an area of the surface. Two sensors separately detect radiation scattered from the surface. One sensor detects radiation scattered in a near-specular region (40-100 milliradians or 2.29-5.73 degrees), while a second sensor detects radiation scattered in a far-specular region (greater than 100 milliradians or 5.73 degrees). The near-specular signal is normalized with respect to the far-specular signal to indicate a pit. However, the '264 and '265 devices lack any means for distinguishing between a surface depression, i. e., a pit, and a surface protrusion, i. e., a bump. The '264 and '265 references also make the assumption that surface depressions or "pits" do not have far-specular reflection patterns, which has proved to be a limiting and problematic assumption. Moreover, the Quackenbos devices lack any means for determining the heights or depths of defects or contaminants, such as bumps, pits and scratches.
Another example of existing defect detection systems include brightfield interferometric systems. In such interferometric systems, light of a particular frequency is reflected off a disk and collected in the near-specular region. The reflected light is then interfered with a reference beam to produce an interference pattern. Although such brightfield interferometric systems can readily detect large surface defects, they cannot easily detect small surface defects. To increase the sensitivity of such brightfield interferometric systems, the size of the illumination spot which is focused on the surface must be decreased. More of the illuminating wavefront is therefore perturbed by the surface defect. This results in less throughput, due to a small diameter illumination spot, but also requires the use of a focus servo to keep the illumination spot in focus. Small illumination spots are formed with large numerical apertures which in turn produce little depth-of-focus.
Thus, there is a need in the magnetic disk drive industry for a non-contact optical inspection instrument which is capable of detecting and providing the profile of defects on the surfaces of polished magnetic disk substrates. This instrument must be sensitive, fast and inexpensive and must be capable of detecting surface defects and estimating the size of these defects. This instrument must also be able to distinguish between various kinds of defects such as bumps, pits and scratches and also between these defects and surface contaminants such as particles and stains. The instrument should also be able to determine the heights and depths of these contaminants or defects while simultaneously providing high sensitivity and throughput.
In optical systems, electro-optic modulators or acousto-optic modulators are typically used in establishing an optical communication link. The information signal is typically impressed electro-optically as an amplitude modulation on a laser beam. Alternatively, the information signal is impressed acousto-optically as a frequency modulation on the laser beam. The signal is subsequently recovered by an optical detector which includes a demodulator.
One common class of Frequency Modulation (FM) demodulators operates by detecting the zero-crossings of the input signal, providing a precise pulse at each zero crossing, and then low-pass filtering these pulses to obtain a voltage that is proportional to the frequency of the input signal. This method is suitable in situations where the modulation frequency is far lower than the carrier frequency, such as in commercial FM receivers, where the carrier frequency is typically 100 MHz and where the maximum frequency modulation or deviation is only about +/-30 KHz. However, if these conventional detectors are used in systems where the modulation frequency is rapidly changing, such as in interferometer systems, the response of the low-pass filter must be very steep. This is necessary to accommodate the high modulation frequency while blocking the undesirable signal energy at the zero-crossing frequency. In addition to being costly, such steep low-pass filters exhibit undesirable phase and group delay variations in the pass-band, making them unsuitable for many applications. For example, in imaging applications, in which rapidly changing spatial surface characteristics are detected as rapidly varying electrical frequency, such group delay variations will cause small objects or sharp edges in large objects to appear to be slightly displaced in position, causing an aberration on the resulting image.
Accordingly, there is a need in the technology for a demodulator which can accommodate high modulation frequencies while blocking undesirable signal energy at the zero-crossing frequency of the modulated signal. The demodulator must also exhibit minimal phase and group delay variations in the pass-band.